You've got to love the rainbow, it's an awesome sight. But I don't want humans to have the pleasure of seeing the rainbow.

What change to the earth, atmosphere and so on, is needed so that humans from approximately 2250 BC do not know of the existence of the rainbow?

Some more details:

Human knowledge/technology between 2500 BC and 2000 BC.

Preferably without affecting flora and fauna too much.

I'm preferably looking for a change that does not affect the human body, such as the eyes (but I find answers exploring this option also intresting).

Reality check based on the earth from about 2250 BC.

Bonus: Preferably without changing the law's of nature, so it should in theory be possible to have this (reality-check).

Bonus: Not only the rainbow, but also other ways of dispersing light should not be known/visible to humans (waterfall + sunlight).

I personally was thinking to change the atmosphere and fill it with lot's of water, so that all the light from stars or the sun is filtered before it reaches earth. But what would happen to plants and animals in this case?

16 Answers
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You need to have direct sunlight that can be dispersed to see a rainbow. If you have only already dispersed light, no rainbow can occur.

If you have a permanently cloudy atmosphere (like it is the case on Venus), there never will be direct sunlight, and thus no rainbows.

If you need a solid physical explanation for it rather than a handwaving "different composition of the atmosphere", the best option would be tiny particles in the air that make it easier for water to condense. In the prehistorical past, massive volcano eruptions have darkened the earth for centuries due to the amount of dirt particles they emitted into the air. Of course, this is extremely detrimental to any form of life, so tune it down a bit and let the volcano eruptions be constant, but less intense, so that the clouds forming in the atmosphere are less thick and allow for enough sunlight to make life possible.

$\begingroup$And you woudn't need it for that long, a generation and a bit and you'll just have some elders muttering about "there being more colour when i was young"$\endgroup$
– BorghMar 14 '18 at 14:33

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$\begingroup$It's made me a little bit sad knowing there are no rainbows on Venus. Poor Venus.$\endgroup$
– RyanfaeScotlandMar 14 '18 at 17:57

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$\begingroup$@RyanfaeScotland With a surface temperature of 460°C, it is also a bit difficult for H2O to exist in its fluid form...$\endgroup$
– ThernMar 14 '18 at 23:53

$\begingroup$@jean No, since you need (more or less) collimated light to generate a rainbow pattern. If there is no sun, diffuse light comes from all directions with roughly the same intensity, which does not allow for rainbows. (Strictly speaking, white light from all directions is diffracted at the water drops, but since it has no predominant direction, all diffractions again add up to white light.)$\endgroup$
– ThernMar 15 '18 at 13:09

People with monochromatic vision can see no colour at all and their
world consists of different shades of grey ranging from black to
white, rather like only seeing the world on an old black and white
television set. Achromatopsia is extremely rare, occuring only in
approximately 1 person in 33,000 and its symptoms can make life very
difficult. Usually someone with achromatopsia will need to wear dark
glasses inside in normal light conditions.

Many cases of colorblindness are genetic. In your world, through a founder effect, humans are colorblind. They will be aware that there are light effects that can be produced by clouds in rain, but they will not know the rainbow because they do not know color.

Monochromatic light

From what I know the rainbow phenomenon is only possible because the Sun light it involves is the superposition of “many” different wavelengths. Basically, the rainbow is just the separation of these wavelengths.

A monocromatic star

So if you don’t want them in your world, you could envisage to turn the Sun into a monochromatic star. These kind of star seems to exist and be known as astrophysical maser (thanks to this stack post !).

EDIT : Unfortunately and thanks to @LSerni, it seems that the phenomenon behind these stars is not as simple at it seems to be and that they should be seen as any other regular "black body" star from their own system point of view... So let's try something else :

An atmospheric filter

Another option is that the light emitted by your star is somehow filtered before hitting the Earth, leading to a monochromatic light. This filtering phenomenon already occurs thanks to the ozone layer, which stops some a part of the UV wavelengths. I’m not an expert, but a change in the atmosphere composition could certainly leads to a better filtering and even maybe to a monochromatic light on Earth…

Of course, this will have a lot of repercussion on the planet.

A Stellar filter

Last possibility, an extra-atmospheric filter. Imagine a very dense stellar cloud that would stand between your planet and its star. As explained here such a cloud, if composed of hydrogen and heated by a very hot star, can lead to the emission of a monochromatic light. I don’t know if this kind of plasma cloud can be dense enough to totally bloc the star light and become the only source of glowing-light for your planet but if so it would certainly lead to an amazing sky!

$\begingroup$Monochromatic light on Earth would have a big impact on fauna and flora. For instance, plants could end up evolving something other than chlorophyll for photosynthesis, depending on the wavelengths available. And camouflage, mating rituals, flower identification etc. would be completely different for animals, to the point that for us the fauna and flora would be completely alien.$\endgroup$
– RenanMar 14 '18 at 13:34

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$\begingroup$@Renan Yes, I totally agree with you on this point. It would effectively changes the fauna and flora on many levels. Pretty sure human species would never even see the day ("see the light" in French).$\endgroup$
– FreedomjailMar 14 '18 at 13:47

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$\begingroup$Unfortunately, there are no "monochromatic stars". There are stars that also emit coherent and collimated microwaves, so that at a large distance (i.e. from here on Earth) you almost only see the microwaves, which arrived intact in a single direction, and not the sunlight, that dissipated in all directions. But from their own system, they would look like normal "black body" stars.$\endgroup$
– LSerniMar 14 '18 at 13:51

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$\begingroup$Monochromatic light would prevent a colored bow, but it would still produce a bow effect. Imagine that only the two orange lines of the sodium spectrum get through: 5890A and 5896A if I remember correctly from high school physics in the 1970's. All this light would land at the arc of a conventional bow that is that specific color. A very similar problem to that of Willk's answer.$\endgroup$
– dgnuffMar 15 '18 at 6:25

Maybe they live in a heavily forested area with a thick canopy and the sky isn't visible. They can find everything they need for life, and outside the forest, there are predators that easily fall humans, so they don't go out. Maybe the forest is extensive enough that they don't find a way out or bordered by sheer cliffs.

Obviously this cannot be true for the entirety of humanity (particularly because one phase of our evolution was supposed to have taken place on savannah) but it could be true for a small group or a collection of tribes.

$\begingroup$Interesting approach, but in theory every little bit of light can cause the rainbow effect (waterfall with some sunlight). I may need to add this detail to the question.$\endgroup$
– Rolf ツMar 14 '18 at 10:52

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$\begingroup$@Rolfツ Rainbow Falls proves you right as well. I'm more familiar with the one in Tennessee, but I would think the sheer abundance of waterfalls sharing this name says a lot. en.wikipedia.org/wiki/Rainbow_Falls$\endgroup$
– PleiadesMar 14 '18 at 13:46

$\begingroup$@Rolfツ true but then you just need to put the people inside a plain or in the middle of a large basin like the Amazon.$\endgroup$
– ErikMar 16 '18 at 16:00

I personally was thinking to change the atmosphere and fill it with lot's of water, so that all the light from stars or the sun is filtered before it reaches earth. But what would happen to plants and animals in this case?

Let's put a water canopy in the upper atmosphere

This will have several interesting effects. First it will work to diffuse light so that there is not enough of the spectrum left to crate a rainbow should it hit water droplets again. It would also allow plants and animal to probably grow bigger and live longer than real-life equivalents.

Water would filter out many of the harmful rays of the sun (alpha and beta radiation for example). A water canopy in combination with the ozone layer would block most of the ultra violet radiations from the sunlight as well. Depending on the thickness of the canopy, you could still allow enough energy to reach earth for everything, while still filtering out the harmful stuff.

To continue down this tangent a bit further, this would also likely increase oxygen content and atmospheric pressure. So everything on earth would be much healthier and heal a lot faster.

Let's boost the humidity until there is no rain

The more humid the air, the more saturated it is will water. If the air is completely saturated with water, then evaporation can no longer occur. If there is no evaporation, the water cycle stops and you have no rain. 100% humidity is not enough to cause rainbows (especially if there is a water canopy diffusing everything), and without rain, there will be no water droplets either (the primary source of rainbows IRL)

If you boost your humidity, any excess water will naturally congregate on the ground every night when things cool down. This, in addition to underground springs, will be able to let your plants grow without the need for rain.

To keep things from getting to hot with your 90-100% humidity, all you need to do is go back to your water canopy. A greenhouse effect will occur that spreads the heat energy evenly throughout the whole globe. Granted you won't have ice cap, but you will have an even, liveable, temperature (unlike IRL where if the ice cap melt everything either becomes a desert or a tundra)

TL;DR

$\begingroup$The theory is that the opposite happened in the Bible with Noah and the flood — that the water from the atmosphere dropped to the earth. According to the Bible, only after the flood was there a rainbow (and that a rainbow is God's promise that he would never flood the earth again).$\endgroup$
– Phil MMar 14 '18 at 18:50

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$\begingroup$@PhilM, that's actually where I got the inspiration for this answer from.$\endgroup$
– amflareMar 14 '18 at 19:09

Rainbows happen because water droplets in the air work like prisms. At just the right distance and With clear air, the refracted light reaches an observer's eye in such a way that the rainbow can be seen.

Let me add a third option: impurities in air. Dust, volcanic ash or other suspended particles other than water droplets may be present in the air in such a way that enough sunlight still reaches the ground for flora to thrive and for people to see, but the farthest distance people would be able to see would be reduced. In addition to that, these particles could break the "prism" effect of water droplets for long distances by spreading light like a light fog would.

You need some way of pre-dispersing the Sun's light without diminishing its overall intensity, essentially "expanding" the Sun's image in the sky from a small, very bright disc to a large shape at least one quarter of the sky in size.

There is no "natural" way of doing it - you'd need a large refraction lattice positioned in low orbit. For short periods (a few thousands of years is probably already too much) you could imagine a very dispersed ring of ice crystals all around the Earth, a sort of super-Kessler apocalypse:

The ring (actually a hollow sphere) should be at least one third of Earth radius thick (I'm not too sure of this figure, I'd need to run some calculations), because it needs to balance the fraction of sunlight that will be reflected away from the Earth with the fraction of sunlight that would not have hit the Earth, but is now reflected towards it.

One possibly harmful consequence of this setup, however, would also be a very bright night.

I can't get my brain round the maths, but I'm wondering if it means you never get the right refraction angles to create the rainbow. i.e. there is never a case where the sun is behind you and the cloud in front because the sun is always directly above.

[Edit]
To frame this in the context of the question: A super advanced species built the Dyson sphere and has long since died out. Millions of years later a human like species evolves from one of the many other creatures in the sphere and never experiences rainbows (or night).

$\begingroup$This does not really answer the question. If you have a different question, you can ask it by clicking Ask Question. You can also add a bounty to draw more attention to this question. - From Review$\endgroup$
– GrahamMar 16 '18 at 12:42

$\begingroup$@Graham Actually, I think this has a point. If the sun is always exactly overhead, like it would be if you were standing on the inside of a Dyson sphere, then it would never be at the right angle to create a rainbow. With a little math, it can be shown that the angle between incoming white sunlight and the outgoing rainbow colors is about 42 degrees. To be more precise, the red light comes out at 42 degrees and blue light comes out at 40 degrees. So if the light is always coming in at a 90 degree angle, rainbows can never form.$\endgroup$
– AndyD273Mar 16 '18 at 13:30

$\begingroup$But then there never would be night, it would always be as bright as noon. That is a massive change far beyond just having no rainbows. But interesting train of thought...$\endgroup$
– ThernMar 16 '18 at 16:05

$\begingroup$@thern A hot Jupiter could cast an eclipse shadow. Depending on the relative sizes of the 3 bodies (sphere, sun, Jupiter) the shadow could be quite large and approximate to a short night/dusk.$\endgroup$
– MartinMar 16 '18 at 16:30

There is a school of thought in Creationism that promotes the Canopy Theory. That basically states that the atmosphere prior to the 40 days/nights of rain that caused Noah's flood wasn't like the one we know now. That's why the Rainbow was such a great sign to Noah and his family - they'd never seen one before.

So simply go back and alter the structure of our atmosphere, and you're there.

I'm not sure this would work as intended, but you just need to remove some air.
As we know: the higher you are the lesser air there is.

So if we could just remove some (actually a lot of) air - still have atmosphere to protect from UV and other stuff - water vapor couldn't get too high and form clouds so there would be no rain and no rain means no rainbow.

If there were no clouds I'd guess that earth would be really foggy and that would also stop you from seeing too far in case rainbow was somewhere. In my opinion fog wouldn't stop plants from growing or animals from surviving tho they'd adapt to the climate and evolve different.

Also if you could somehow get fog to last forever 500 years it could be solution all by itself

$\begingroup$@Renan It's when you look at them from top, also I was talking about lowering air level, why are you talking about being higher?$\endgroup$
– NoOorZ24Mar 14 '18 at 13:28

$\begingroup$My bad, I used a bad example to make my point. Anyway, thinner air (which is what you get with "less air" means the light will spread less between water droplets and your eyes, so the rainbow will be sharper. A regular rainbow will look much sharper if seen from atop a mountain.$\endgroup$
– RenanMar 14 '18 at 13:47

$\begingroup$I'm talking about more extreme case where water vapor wouldn't get high enough to form clouds, to make it rain but would simply form a thick fog around the world. light would be just scattered and couldn't form rainbow at first place$\endgroup$
– NoOorZ24Mar 14 '18 at 13:59

Visible light is part of the electromagnetic spectrum. https://imagine.gsfc.nasa.gov/science/toolbox/emspectrum1.html The wavelength of electromagnetic energy is inversely proportional to the frequency of the electromagnetic energy. This wavelength - frequency relationship is true because electromagnetic energy travels at the speed of light (for a specific medium) regardless of the frequency. Red light has a longer wavelength than violet light.

However, when it comes to light traveling through water, different frequencies (colors) travel at different speeds. That is why different colors refract at different angles when traveling through clear solids and liquids.

In my mind, the speed of light should be constant. That it isn't constant in water seems to be a change in the laws of nature.

Sorry that I didn't stick with your "laws of nature" constraint, but I find it illogical and unnatural that light behaves this way.

$\begingroup$I actually like this answer. It doesn't seem like a big change, although it will probably trigger a butterfly effect.$\endgroup$
– Rolf ツMar 14 '18 at 20:22

$\begingroup$It is not illogical. Light in matter still has the same speed as in vacuum, but the photons are permanently absorbed and re-emitted by the electrons of the matter, which effectively slows down their overall mean speed.$\endgroup$
– ThernMar 15 '18 at 14:57

$\begingroup$The constant c specifically represents the speed of light in a vacuum, so it's not really inconsistent.$\endgroup$
– barbecueMar 15 '18 at 15:35

$\begingroup$I acknowledge that light travels slower through a medium than a vacuum. What does not make sense is that red light in water does not travel the same speed as blue light in water.$\endgroup$
– dadinckMar 16 '18 at 2:15

I would say (based on my limited knowledge of rainbows) that you would have to make the sun be always almost directly overhead, so that the rainbow would only appear underneath people (unlikely). Or, you could lower the humidity of the atmosphere, or even force your civilization into a canyon-like area, where there isn't much to see beyond a couple miles.

You don’t need to prevent rainbows. You just want to prevent surface-dwelling humans from noticing rainbows. So let’s add a new factor. I suggest a small, unpleasant, flying insect with an unusual life cycle. They only mate when there is moist air and a light source present. When both of these conditions occur, they immediately take to the sky. And there’s zillions of these things. They are literally everywhere. And they bite, or sting, or smell bad, or tend to fly up your nose, or all of the above. They have been around for longer than the humans. So every human knows to cover their head/get inside a house or cave whenever there is a break in the rain. Because the sky is about to become absolutely clogged with those obnoxious pests. This gives you three reasons why nobody has ever seen a rainbow:
1. humans actively avoid the sky when conditions are right.
2. So many bugs are in the sky, you can’t see the rainbows anyway.
3. even if some rays of light make it through the bugs, the humans are probably so preoccupied with shooing away biting insects that they aren’t going to notice the refracted light.

The arc of a rainbow is caused by the curvature of the Earth. If flat Earth was even remotely possible (it really isn't flat Earther's) then there would be no bow, possibly just straight lines.

In response to monochromatic sun's, how about a red giant star where the Earth was still in the Goldilocks zone. I am fairly sure you still get rainbows, only in 1 colour. Laser light still refracts and that is highly monochromatic.

Multiple suns could prevent you seeing rainbows. If the area was drowned out by light from different angles there would be no apparent rainbow. I have no idea of the solar system that would mean a planet always sees 2+ suns at all habitable locations but Issac Asimov tells a story of a society that never sees darkness in Nightfall

$\begingroup$Can you cite a source for your claim that the curve of a rainbow is caused by the curvature of the earth?$\endgroup$
– sphenningsMar 14 '18 at 16:39

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$\begingroup$A rainbow is a circle regardless of the shape of the earth. On our Earth the lower part is usually cut off, because the Earth is in the way. But that would also be true on a flat earth. You can see the full circle e.g. from a plane.$\endgroup$
– GraipherMar 14 '18 at 16:40

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$\begingroup$Sorry, but not much right here. Rainbows are always round (the shape of the drops is the reason), a red giant star is not monochromatic (its mean wavelength is just red, not yellow or green), and multiple suns would just allow for multiple simultaneous rainbows.$\endgroup$
– ThernMar 15 '18 at 7:23

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